1. Field of the Invention
This invention pertains to improvements in timing and control circuits, and, more particularly, to externally tunable timing circuits, and methods for operating such circuits. The invention also pertains to improvements in circuits and methods for synchronizing a plurality of timers, and to improvements in detonators for explosives, or the like.
2. Technical Background
A variety of applications exists for timing control systems. This is especially true in those instances in which a control unit starts one or more timing circuits that generate a signal after a predefined delay period. The delay period may be individually programmed into each circuit through the control unit. The timing circuits are often located at an operation site selected to reduce reliability problems that can result from interference or physical stress. Systems are often formed with multiple timing circuits, and are useful to control time sequenced operations, particularly in performing the individual operations at different locations.
In many systems applications, such as those involving generating acoustic or vibration waveforms, control of the sequence of operations to accuracies ranging between milliseconds and microseconds often may be necessary. The stability of the oscillator upon which clock signals are based in each timing circuit is important to the accuracy of the event timing. To achieve this reliability, crystal oscillators can be used in many applications. While crystal oscillators are normally accurate, they are generally unsatisfactory for operation in harsh vibration environments, such as those experienced, for instance, in the blasting industry. On the other hand, the performance of a voltage controlled oscillator (VCO) is normally stable by high impact shock waves of the type generated by explosive charges. Nevertheless, circuits that incorporate VCOs are known to experience rapid frequency drifts.
In theory, a solution to the problem of frequency drift in a time delay system formed with VCOs should entail calibration of either the clock signal or the delay time before the control unit activates the timing circuit. Efforts to carry out this approach have had limited success. In systems that include a large number, for example, 100, timing circuits, the time required to input delay information under calibrated conditions can be long enough to allow some of the VCOs to drift.
Improved time accuracies are being sought to provide greater control over blasting operations, and to improve the quality of data gained, for example, for seismic or similar analyses by the detonation of explosive charges in predetermined time sequences. Also, in blasting operations, an improved accuracy in timing successive explosions can lead to greater control over ground vibration and the fracture of rock formations. For example, in some instances it is desirable to control blasting detonations to provide reinforcing shock waves, improving blasting efficiency. In other instances, it may also be desirable to generate secondary accurate shock waves to create a canceling effect on the propagation of other explosive blasts. This nulling can be applied, for example, to limit damage due to ground vibrations propagating from the locus of blasting into remote, but sensitive, regions.
For example, U.S. Pat. No. 4,419,933 shows a system arrangement in which a central unit sequentially provides reference timing signals to some timing and load starting devices. The timing signals provided to each device define a delay that is to precede starting a device. Each delay defined by the timing signals is counted with a VCO clock signal generated on the corresponding device.
Thus, programming of the delay time can be done by counting oscillator clock pulses during a timed period corresponding to the desired delay. After receiving a fire command from the control unit, each device generates the predefined delay period by counting out the same number of pulses. Accuracy of this technique for implementing a programmable delay is, of course, dependent upon the stability of the clock frequency between the times at which programming begins and all operations have been started.
When many devices are serially programmed for delay periods ranging up to several seconds, the programming process can be lengthy. For example, when a thousand devices are programmed with varying delay periods, assuming the delay periods range from zero to 3 seconds with an average delay of 1.5 seconds, the time required for programming can be as high as 25 minutes. The cumulative drift among the VCO clocks during a 25 minute period would make it difficult to maintain millisecond accuracy in the delay sequence, and would make microsecond accuracy unattainable.
In many applications, such as those involving detonation of explosive charges in which a number of timer circuits are employed, and in which critical timing relationships among the timers is desired to be maintained, often the destruction of one of the timers causes loss of synchronization or coordination among the remaining timers. This may be especially true in those instances in which subsequent timers base their initial timing on previous timers. Also, in such instances where successive timing is "in series" from one timer to the next, any timing errors in preceding timers results in similar timing errors in successive timers.